Various types of plug valves, such as cylindrical plug valves, ball valves and tapered plug valves have been utilized for a significant period of time. Tapered plug valves are favored in many service conditions because of the closeness of fit that can be obtained between the tapered plug member and the tapered valve chamber surfaces. Tapered plug valves offer the capability for achieving close sealing tolerances without the introduction of significant torque to achieve valve operation. Tapered plug valves are available in lubricated models where sealant material provides for lubrication of internal components and assists in the sealing activity of the valve. The injected lubricant minimizes leakage in the clearance between the tapered plug member and the internal tapered sealing surface of the valve body. Tapered plug valves are also available in non-lubricated models where sealing is affected by the closeness of fit between the plug and body surface.
Tapered plug valves are at times disadvantageous because pressure induced locking may occur especially under the influence of positive and negative pressure transient conditions. In cases where surges in line pressure occur or where line pressure suddenly increases or decreases, the tapered plug member can be moved axially under pressure influence to a tightly wedged position within the apex tapered portion of the valve chamber. In this tightly wedged position, rotation of the plug member may not be possible or excessive torque may be required to move the plug member between its open and closed positions. The relationship of the plug member to internal valve chamber walls typically defines chambers at each axial end of the plug members. When line pressure changes occur, a pressure differential exists between these internal chambers which becomes balanced at the end of a "transient period." The transient period is that period of time during which there is a transfer of fluid through the clearances between the plug and body to the internal chambers due to the pressure differential. Pressure transients are "positive" when line pressure increases causing leakage flow from the flow passage to the internal chambers and "negative" when line pressure decreases, causing leakage flow from the internal chambers to the flow passage. It is desirable therefore to provide means for preventing pressure responsive movement of the plug member to a locked position within the valve chamber during either positive or negative pressure transients.
As temperature transients occur the valve body may increase in effective dimension thereby enabling the tapered plug member to move further towards the small end of the valve chamber. Upon subsequent decrease in valve chamber dimension, caused by cooling by the valve body, the valve chamber surfaces of the valve body may seize the plug the lock it against rotation. It is desirable of course to eliminate the possibility of plug movement toward its apex and thus prevent locking of the plug member due to the effects of temperature transients.
Taper locking can also occur under circumstances where the valve is mounted with the large base portion of the plug member positioned above the small apex portion of the plug. The weight of the plug can cause it to descent toward its taper apex by gravitational force and in time it can become locked against normal rotation.